1. Field of the Invention
This invention relates to an apparatus for driving cooling fans.
2. Description of the Related Art
In hydraulic drive equipment such as construction machinery, a hydraulic pump is driven by the engine, and actuation pressure oil discharged from the hydraulic pump is supplied to hydraulic actuators such as hydraulic cylinders through control valves, whereby working equipment is activated.
The engine and actuation pressure oil must be cooled.
The engine is mainly cooled by a water-cooling type cooling system. That is, cooling is performed by circulating a coolant (cooling water) through a water jacket provided in the engine block. Coolant that has become hot inside the water jacket is led to a radiator and there cooled, whereupon the cooled coolant is returned back to the water jacket.
The cooling of the actuation pressure oil is performed by routing the actuation pressure oil to an oil cooler. The energy loss occurring inside the hydraulic circuit is conducted as heat to the actuation pressure oil. Similarly to the coolant, the actuation pressure oil is routed to the oil cooler and there cooled, whereupon the cooled actuation pressure oil is returned back to the hydraulic circuit.
Both the radiator and the oil cooler are cooled by blown air generated by a cooling fan. In most cases, the oil cooler and radiator are placed in order in the path of the blown air generated by the cooling fan. Cooling efficiency is always considered in the specific arrangement therefor.
The cooling fan is attached to the engine drive shaft. For this reason, the cooling fan rotational speed (hereinafter referred to as R.P.M.) will correspond to the engine R.P.M.
In recent years there has been a demand for noise reduction in construction equipment. For this reason, engines are shielded as a noise-reduction measure against the noise generated by the engines. However, when the engine is shielded, it is not possible to attach a cooling fan to the engine drive shaft.
Thereupon, the technology seen in Japanese Patent Application Laid-Open No. 250342/1997, as published, is being adopted.
An invention is described in this patent publication wherein a fixed-capacity hydraulic pump for driving a fan and a fixed-capacity hydraulic motor for driving a fan are installed, separate from the engine, the pressure oil discharged from the fixed-capacity hydraulic pump for driving the fan is supplied to the fixed-capacity hydraulic motor for driving the fan, and the cooling fan is driven.
In this case, the fixed-capacity hydraulic pump is provided as a dedicated hydraulic pump for driving the fan. And, when the pressure that is applied from the dedicated fixed capacity hydraulic pump for driving the fan to the fixed-capacity hydraulic motor for driving the fan declines, control is performed by a changeover valve to replenish-supply pressure oil to the fixed-capacity hydraulic motor for driving the fan, without delay, from a tank.
The technology diagrammed in FIG. 6 is also public knowledge.
The apparatus diagrammed in FIG. 6 is an apparatus for driving a cooling fan 130, wherein a variable-capacity hydraulic pump 110 for driving a fan and a fixed-capacity hydraulic motor 120 for driving the fan are installed separately from the engine 100, and pressure oil discharged from the variable-capacity hydraulic pump 110 for driving the fan is supplied to the fixed-capacity hydraulic motor 120 for driving the fan.
In this case, a swash plate 110a of the variable-capacity hydraulic pump 110 is driven by a servo piston 160. The servo piston 160 is driven in response to the flow volume of pressure oil supplied from control valves 140 and 150. A relief valve 170 is deployed in a pipeline communicating with the control valves 140 and 150. The pressure set in this relief valve 170 varies according to the temperature inside the cooling water path of the engine 100. Accordingly, the control valves 140 and 150 are driven in response to the temperature in the cooling water path of the engine 100, so as to vary the swash plate 110a of the variable capacity hydraulic pump 110.
As based on either the invention described in the patent publication noted above or the technology diagrammed in FIG. 6, a cooling fan is driven, using as the drive source therefor a hydraulic pump that is separate from the engine. For this reason, the degree of freedom in positioning the cooling fan, radiator, oil cooler, and other equipment is increased, and both engine shielding and cooling by a cooling fan are realized. There are problems therewith, however, such as those noted below.
In the first place, the technology described in the patent publication cited above merely performs flow-volume replenishment control. This technology does not effect precise control such that the volume of blown air from the cooling fan (i.e. the R.P.M.) becomes an R.P.M. that is optimum for cooling.
The technology represented in FIG. 6, moreover, controls the swash plate of the hydraulic pump in response to changes in the pressure set in the relief valve 170. In such cases, it is difficult to effect precision control so that the air volume from the cooling fan (i.e. R.P.M.) becomes an optimum R.P.M. for cooling in response to changes in the pressure set in the relief valve 170.
Hence it is not possible with the prior art to precisely control the volume of blown air from the cooling fan (R.P.M.).
Furthermore, the fixed-capacity hydraulic pump for driving the fan described in the patent publication cited earlier is provided exclusively for the fan drive. Accordingly, it is therewith necessary to install another hydraulic pump for that purpose, resulting in an increase in the number of parts. This gives rise to a demand, in the case of driving the cooling fan with a hydraulic pressure source separate from the engine, for building the hydraulic circuit with fewer parts. This demand, however, cannot be satisfied with the prior art.
In its particulars, moreover, the control described in the patent publication cited above does nothing more than supply/replenish pressure oil, without delay, to the fan-drive fixed-capacity hydraulic motor from a tank, thus preventing cavitation.
Therewith, under conditions where the torque absorbed by the fan-drive hydraulic motor fluctuates, the cooling fan R.P.M. fluctuates also and its turning becomes unstable. As based on the invention described in the patent publication cited above, when the torque absorbed by the fan-drive hydraulic motor fluctuates, it is not possible to suppress fluctuations in the cooling fan R.P.M. and thus stably control the turning thereof.
Under conditions also wherein the load on the fan-drive hydraulic motor fluctuates, the cooling fan R.P.M. fluctuates also and its turning ceases to be stable. As based on the invention described in the patent publication cited earlier, when the load on the fan-drive hydraulic motor fluctuates, it is not possible to suppress fluctuations in the cooling fan R.P.M. and thus stably control the turning thereof.
That being so, an object of the present invention is to precisely control the blown air volume from the cooling fan (R.P.M.).
Another object of the present invention is to construct the hydraulic circuit with fewer parts in cases where the cooling fan is driven by a hydraulic pressure source.
Yet another object of the present invention is, when the torque absorbed by the fan-drive hydraulic motor fluctuates, to suppress fluctuations in the cooling fan R.P.M. and stably control the turning thereof.
Still another object of the present invention is, when the load on the fan-drive hydraulic motor fluctuates, to suppress fluctuations in the cooling fan R.P.M. and stably control the turning thereof.